This invention relates in general to electrical machine systems and in particular to the location of Hall effect sensors relative to the stator assembly of a two-pole toothless permanent magnet brushless dc machine.
A brushless dc motor system typically consists of a motor, a solid-state inverter, a controller and a rotor position sensing system. The motor includes a permanent magnet rotor and a stator whose polyphase power windings surround the rotor. Under command from the controller, the inverter energizes selected phase windings at the correct time and sequence to generate a stator mmf field that reacts with the rotor's flux field, causing the rotor to rotate.
The rotor position sensing system generates with signals indicating the position of the rotor relative to the phase windings. The controller decodes these position signals into commutation commands which command the inverter to energize the phase windings at the correct time and sequence.
The rotor position sensing system can include proximity probes and a back emf generator. During motor startup, the rotor position signals are generated by the proximity probes. Once the motor reaches roughly ten percent of maximum rated speed, however a switch is made to the back emf generator which generates the rotor position signals.
The proximity probes sense the position of a cam that is mounted about one end of the motor's shaft, external to the motor. However, this design has the unwanted effect of increasing the axial length of the motor. Further, the proximity probes must be adjusted for precise commutation.
Instead of using proximity probes, the rotor position sensing system can use Hall effect sensors. The Hall effect sensors are mounted underneath stator end turns where they work off rotor end leakage flux. The use of Hall effect sensors overcomes the problem of axial length. For small machines, however, the end leakage flux may not be sufficient for the Hall effect sensors to operate reliably.